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How geological variations govern local expertise this month focuses on Devon and Cornwall

During the Pleistocene, Devon and Cornwall were beyond the maximum extent of the ice sheets. The area experienced cold-climate periglacial conditions and was affected by periodic lowering of sea level, when huge quantities of water were locked up in the advancing ice sheets.

'That the area was not glaciated is the most important factor influencing the engineering geology,' says John Grimes, who heads Ivybridge based John Grimes Partnership. 'Tertiary weathering gives a multiplicity of ground conditions from deep fluvio deposits to rotten granites in locked sand.'

Many of the main towns in Devon and Cornwall are on tidal estuaries, formed by 'drowning' of the lower reaches of river valleys by the post- glacial rise in sea level. 'The drowned valleys are infilled by thick deposits of geologically recent alluvial and estuarine sediments, typically clay, silt sand and peat,' adds John Harris, director of Frederick Sherrell.

These sediment-filled valleys have created corridors for transport links and attractive sites for development and so it is here that much of the geotechnical work takes place. Development of these sites often requires use of ground improvement and piling techniques because of the great thickness of compressible soil.

This came as a surprise to James Woodward of Anglian Site Investigation which set up an office in Plymouth last autumn.

'We assumed we would be dealing mostly with the hard rock geology, but in reality there isn't a big difference to the drilling techniques we use in the south east,' he says.

Investigations for new houses tend to be on clay sites (but rarely heavily overconsolidated as the region was not glaciated), while house subsidence problems are usually related to leaking drains or water mains in fine granular silts and sands.

In hard rock areas foundations are, understandably, not generally an issue. Of more importance here is risk of collapse from mining. In the worst cases, such as the widely reported Gunnislake collapse (Ground Engineering October 1993), deep shafts close to housing have collapsed dramatically and suddenly.

Mining records are reasonably comprehensive, even if not accurate in detail, and mine record searches are standard during house conveyancing. However there is a sizeable industry that locates and stabilises abandoned mine workings.

Another problem created by old mine-working is the risk of contamination of soil and groundwater, particularly by arsenic associated with mine wastes.

Nowhere in Cornwall and Devon is more than 40km from the sea, and given the young stage of shoreline evolution, the region boasts a number specialists in coastal engineering and management, and investigation of nearshore and inshore sites.

Naturally occurring radon gas is often assumed to be an issue in the region, but is not considered to have much mileage locally.

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